The invention relates to a continuous adsorptive separation process used to separate chemical compounds such as C8 aromatic hydrocarbons. The invention specifically relates to an innovative fractional distillation method which reduces the cost of recovering desorbent from the extract stream of a continuous adsorptive separation process.
In many commercially important petrochemical and petroleum industry processes it is desired to separate closely boiling chemical compounds or to perform a separation of chemical compounds by structural class. It is very difficult or impossible to do this by conventional fractional distillation due to the requirement for numerous fractionation columns which may consume excessive amounts of energy. The relevant industries have responded to this problem by utilizing other separatory methods which are capable of performing a separation based upon chemical structure or characteristics. Adsorptive separation is one such method and is widely used to perform these separations.
In the practice of adsorptive separation a feed mixture comprising two or more compounds of different skeletal structure is passed through one or more beds of an adsorbent which selectively adsorbs a compound of one skeletal structure while permitting other components of the feed stream to pass through the adsorption zone in an unchanged condition. The flow of the feed through the adsorbent bed is stopped and the adsorption zone is then flushed to remove nonadsorbed materials surrounding the adsorbent. Thereafter the desired compound is desorbed from the adsorbent by passing a desorbent stream through the adsorbent bed. The desorbent material is commonly also used to flush nonadsorbed materials from the void spaces around and within the adsorbent. This could be performed in a single large bed of adsorbent or in several parallel beds on a swing bed basis. However, it has been found that simulated moving bed adsorptive separation provides several advantages such as high purity and recovery. Therefore, many commercial scale petrochemical separations especially for specific paraffins and xylenes are performed using simulated countercurrent moving bed (SMB) technology.
The passage of the desorbent through the adsorbent dislodges the selectively retained compounds to produce an extract stream. The extract stream contains a mixture of desorbent and the desired compounds, with these materials being then separated by distillation in a column referred to as the extract column. If more than one compound is retained on the adsorbent and removed as part of the extract, then it is necessary to perform yet another fractionation in a finishing column. The subject invention is aimed at improving the economics of the fractionation employed in recovering the final desired compound from the extract stream.
Several economic advantages are derived from the continuous, as compared to batch-wise, operation of a large scale adsorptive separation processes. Recognition of this has driven the development of simulated moving bed (SMB) adsorptive separation processes. These processes typically employ a rotary valve and a plurality of lines to simulate the countercurrent movement of an adsorbent bed through adsorption and desorption zones. This is depicted, for instance, in U.S. Pat. No. 3,205,166 to D. M. Ludlow, et al. and U.S. Pat. No. 3,201,491 to L. O. Stine et al.
U.S. Pat. No. 3,510,423 to R. W. Neuzil et al. provides a depiction of the customary manner of handling the raffinate and extract streams removed from an SMB process, with the desorbent being recovered, combined and recycled to the adsorption zone. U.S. Pat. No. 4,036,745 describes the use of dual desorbents with a single adsorption zone to provide a higher purity paraffin extract. U.S. Pat. No. 4,006,197 to H. J. Bieser extends this teaching on desorbent recycling to three component desorbent mixtures.
U.S. Pat. No. 5,177,295 issued to A. R. Oroskar et al describes the fractionation of a xe2x80x9cheavyxe2x80x9d desorbent used in the recovery of paraxylene from a mixture of aromatic hydrocarbons.
The dividing wall or Petyluk configuration for fractionation columns was initially introduced some 50 years ago by Petyluk et al. A recent commercialization of a fractionation column employing this technique prompted more recent investigations as described in the article appearing at page s14 of a Supplement to The Chemical Engineer, Aug. 27, 1992.
The use of dividing wall columns in the separation of hydrocarbons is also described in the patent literature. For instance, U.S. Pat. No. 2,471,134 issued to R. O. Wright describes the use of a dividing wall column in the separation of light hydrocarbons ranging from methane to butane. U.S. Pat. No. 4,230,533 issued to V. A. Giroux describes a control system for a dividing wall column and illustrates the use of the claimed invention in the separation of aromatics comprising benzene, toluene and orthoxylene.
The invention is an improved simulated moving bed adsorptive separation process characterized by the use of an integrated fractional distillation column to separate a stream comprising two extract components plus the desorbent into three product streams in only a single fractionation column. That is, the three-component extract stream of the adsorptive separation zone is separated into three high purity process streams in a single dividing wall column. A portion of the column is divided by a vertical wall into parallel fractionation zones with one receiving the extract stream and the other delivering a product stream of the adsorptive separation zone. The desorbent is preferably rejected from the bottom portion of the column. A light product may be recovered overhead. This reduces the capital and operating costs of the required separation and thus of the adsorption process.
One broad embodiment of the invention may be characterized as a simulated moving bed adsorptive separation process which comprises passing a feed stream comprising a first, second and third chemical compounds into an adsorption zone comprising a bed of a selective adsorbent maintained at adsorption promoting conditions under which the first compound is selectively retained on a quantity of the selective adsorbent compared to the second compound, with the third compound having a boiling point sufficiently different from the first and second compounds to allow its facile separation by fractional distillation and with the third compound being adsorbed onto the adsorbent to a lesser extent than said first chemical compound, and thus forming a raffinate stream comprising the second compound and a desorbent formerly present in the quantity of the selective adsorbent; passing a desorbent stream comprising a desorbent compound into contact with said quantity of the selective adsorbent, which has retained the first chemical compound, under desorption promoting conditions to yield an extract stream comprising the desorbent compound, the first compound and the third compound; passing the extract stream into a dividing wall fractionation column operated at fractionation conditions and divided into at least a first and a second parallel fractionation zones by a dividing wall, with the first and the second fractionation zones each having an upper and a lower end located within the fractionation column, with the first and second fractionation zones being in open communication at their upper ends with an undivided upper section of the fractionation column and in open communication at their lower ends with an undivided lower section of the fractionation column, and with the extract stream entering the column at an intermediate point of the first fractionation zone; removing an extract product stream comprising the first compound from an intermediate point of the second fractionation zone; recovering a product stream comprising the third compound from a first end of the fractionation column, removing a process stream comprising the desorbent compound from a second end of the fractionation column.